Active optical fibers, typically doped with erbium, are more and more often used in communication systems. In particular the use of erbium doped active optical devices, and specially of erbium doped fiber amplifiers doped is rapidly growing because of the peculiar passband and gainband characteristics of these devices that favorably coincide with the wavelengths of so-called third window, in the vicinity of 1.5 .mu.m, which is more and more used in optical fiber communication systems.
One of the parameters that most affect doped optical fibers remains the concentration and distribution within the vitreous matrix, generally silicic, of dopant ions, in particular within the core of the optical fiber.
The dopant ions act as active centers similarly to what happens in lasers and therefore their distribution inside the matrix is fundamental in determining the performance of the active optical device.
In optical devices such as fiber amplifiers, the control of the distribution of dopants in the glassy matrix of the core is even more critical because in an optical fiber amplifier the smoothing (levelling) effect of the electromagnetic field due to optical feedback from cavity mirrors does not exist, whereas such effect does exist in lasers.
The article "Chlorine concentration profiles in silica fibers" by H. Hanafusa et al., ELECTRONIC LETTERS, Feb. 16, 1984, UK, vol. 20, no. 4, pages 178-179, and the article "Investigation of the structure of perform materials and fiber-optical waveguides utilizing quartz glass doped with germanium and boron", by A. N. Gur'yanov et al., KVANTOVAYA ELEKTRONIKA, Moskva, October 1979, USSR, vol. 9, no. 10, pages 1238-1242, describe methods for characterizing the doping of the core of an optical fiber and of a preform, respectfully, by x-ray microanalysis. These methods do not provide a direct and reliable distribution pattern of dopant ions on a plane normal to an optical fiber axis.
The determination of the concentration profile, that is, a significatively precise characterization of the doped optical fiber continues to represent a serious problem in view of the difficulty to reliably make such a characterization and this represents a non negligible drawback for the commercial evolution of these technologies.
Until now the characterization is carried out in two different fabrication steps of a doped optical fiber. A first characterization is undertaken on a rod or preform having a diameter of a few centimeters, before the material is drawn in a continuous small diameter fiber. The characterization is made inductively, by exciting the material with a laser and measuring the intensity of the reemission.
However, the drawing process takes place at a temperature of about 800.degree. C. and this inevitably determines a certain diffusion of the doping material during this manufacturing step. Therefore, it is necessary to repeat the characterization on the finished optical fiber, also in this case inductively, and to adapt consequently the length of the active optical fiber in order to attain the required gain value requested by the specifications.